build-sys: do not install ctrl-alt-del.target symlink twice

It was a harmless but pointless duplication. Fixes #4655.

Note: in general we try to install as little as possible in
/etc/systemd/{system,user}. We only install .wants links there for units which
are "user configurable", i.e. which have an [Install] section. Most our units
and aliases are not user configurable, do not have an [Install] section, and
must be symlinked statically during installation. A few units do have an
[Install] section, and are enabled through symlinks in /etc/ during
installation using GENERAL_ALIASES. It *would* be possible to not create those
symlinks, and instead require 'systemctl preset' to be invoked after
installation, but GENERAL_ALIASES works well enough.